Both Plato and Aristotle see a connection between two unique features of human beings: their erect posture and their ability to think. Moreover, both philosophers think that human beings have the erect posture precisely for the sake of thinking. However, their explanations are very different. I present the two explanations and indicate both Aristotle's debts and fundamental disagreements with Plato. It is argued that Aristotle's explanation should be read as tacit criticism of Plato's theory. Although both explanations are plainly wrong by modern standards, I show why Aristotle's explanation is superior to Plato's.